Describe third nerve palsy with its causes and management.

Anatomy and Clinical Features

• The oculomotor (third) nerve supplies the superior, medial and inferior recti, the inferior oblique, the levator palpebrae superioris, and — through its parasympathetic component — the sphincter pupillae and ciliary muscle
• Its course — from the midbrain, between the posterior cerebral and superior cerebellar arteries, alongside the posterior communicating artery, over the petroclinoid ligament, through the lateral wall of the cavernous sinus and the superior orbital fissure within the annulus of Zinn, dividing into superior and inferior divisions
• A complete third nerve palsy gives — complete ptosis (which may mask the diplopia and is often the presenting complaint); the eye "down and out", from the unopposed action of the lateral rectus and superior oblique; a dilated, fixed pupil; and loss OF accommodation
• The pupil rule IS the central fact, and it rests on anatomy: the parasympathetic pupillomotor fibres run peripherally and superficially in the nerve, nourished by the pial vessels.
• Compression therefore affects the pupil early — a posterior communicating artery aneurysm presses from outside and catches the superficial fibres first
• Ischaemia (microvascular) spares the pupil — diabetes and hypertension infarct the core, leaving the superficial pupillomotor fibres intact
• A painful third nerve palsy with pupil involvement is a posterior communicating aneurysm until proved otherwise, and requires urgent CT or MR angiography the same day — because the aneurysm may rupture and the mortality of a subarachnoid haemorrhage is high
• The rule is not absolute: a small proportion of aneurysms spare the pupil initially, and a small proportion of diabetic palsies involve it partially — so in practice most complete third nerve palsies are imaged, and certainly all those under 50